After the end of the 4th week all of the placental villi are tertiary villi. Histologically and morphologically they develop themselves further in the direction of a new type of villus that is better adapted to the growing embryonic needs. The tertiary villi, with a diameter between 60 -200 mm, are homogenous and rich in mesenchyma with few capillaries. They grow first in length and later, through proliferation of the trophoblast and extra-embryonic mesoblast (EEM) as well as of endothelial cells, they also increase in density and width. The trophoblast covering forms numerous trophoblast buds, which at the beginning are formed by the syncytiotrophoblast (similar to the primary villi), but nevertheless cytotrophoblast and the EEM grow very rapidly into these villi.
After the ninth and up to the 16th week the tertiary villi mesh and change through proliferation of the terminal villus mesenchyma, the trophoblast and the fetal endothelial cells in order to finally form intermediate immature villi. The latter generate trophoblast buds, out of which new, thinner villi arise. These villi are rich in large diameter vessels, guaranteeing increased perfusion.
After the 26th week, the (mesenchymatous) tertiary villi develop longer and thinner villi (60 to 200 mm in diameter) in which the fetal capillary network gets richer. These are termed mature intermediate villi that, for their part, form small meshes, terminal or free villi (diameter 40 - 80 mm), as well as new trophoblast buds.
At the end of the pregnancy six types of villi are to be found in the placenta:
- Stem villi
- Tertiary mesenchymatous villi
- Immature intermediate villi
- Mature intermediate villi
- Terminal or free villi
- Trophoblast buds
The stem villi guarantee the mechanical stability of the villus tree, the immature intermediate villi are the place of the proliferation of the trophoblast and the trophoblast buds. Tertiary villi (mesenchymatous) are responsible for the lengthening of the villus tree.
From the mature intermediate villi the free villi or terminal villi arise which makes up 40% of the volume and 50% of the exchange surface of the placenta before birth. (17)